Materials, such as thermoplastic resins, have been used to form pipes including, for example, those having a multilayer design. In some cases, the materials are heated, melted, or extruded, such as through the use of a die having a predetermined shape. Melted resin is provided from a heated source to one or more distributors where the resin is urged into an annular shape.
Traditionally, multilayer pipes are made by extrusion dies having a number of distributors corresponding to the number of layers, or walls, in the pipe. Such dies generally involve the use of a separate plastic extruder for each layer. For example, a dual-wall extrusion die might have two side-feed distributors, each having a corresponding extruder, which is adjusted to control the level of flow for each wall.
The prior art extrusion die designs suffer from several deficiencies. For instance, the use of more than one extruder and distributor increases the number of parts and the cost of production and repair. In extrusion dies which split the flow close to the material exit, there is very little control over the resulting ratio of division. Specifically, because there is a short flow distance between the split and the material exit, there is little resistance to flow. This causes undesirable sensitivity to changes in material properties due to various parameters, such as time, temperature, pin and bushing changes, and switches between raw material lots. This sensitivity results in undesired changes in the ratio of the volumetric flow rate to each layer. Moreover, existing extrusion die designs having a single distributor fail to adequately control material flow properties, such as shear rate and shear stress. As a result, the end product is produced at higher temperatures and pressures, and has greater material thickness variation. Higher temperatures and pressures result in increased production costs. Greater material thickness variation necessitates an increase in raw material usage and, therefore, results in further increases in production costs. The inability to adequately control material flow properties in the prior art thus reduces the efficiency and increases the cost of the extrusion process.
Accordingly, there is a need for an improved extrusion die apparatus and process for improving the efficiency and cost of the extrusion process.